Oceanic crust is denser than continental crust because it is made of basaltic rock. Basaltic rock is denser than silicate rocks which make up continental crust.
For this reason, it can be said that the continental crust is more 'buoyant' than the oceanic crust. Thus, when the two crusts meet at a subduction zone and collide, one of them has to give way. The denser crust ends up subducting (giong underneath) the less dense crust, and the continental crust gets pushed up to form mountain ranges like the Andes or the Rockies.
